In present residential heating, ventilating and air conditioning systems, a 24 volt alternating current (AC) transformer provides low voltage power source for actuation of various components of the HVAC system. A wall thermostat within the space typically receives 24 volt AC, and transmits a 24 volt AC signal via a wire to a heating controller to request or activate heating, or via a second wire to an air conditioning compressor contactor to request or activate cooling. A 24 volt AC signal for requesting heating may be used to directly turn on a 24 volt AC electromechanical device to actuate heating, or the signal may be interpreted by the heating controller as an “on” signal. The 24 volt AC signal may be interpreted as “on” or “off” depending on the presence or absence of the 24 volt AC signal. For each additional thermostat command for other system components, such as a fan, heat pump, humidifier, etc., typically at least one new wire is required for the thermostat to transmit a 24 volt AC signal to the additional component. In the new construction of a building, the amount of wiring installed from the thermostat through the walls to the HVAC system components will accordingly depend on the complexity of the particular HVAC system. An HVAC system typically requires wiring for components such as a furnace controller, an air conditioning compressor relay or contactor, and a fan, but may require additional wiring for a heat pump or staged heating, staged cooling, a humidifier, an air cleaner, or other components. Once the construction is completed and the wiring is in place, subsequent replacement of the HVAC system can be affected by the number of existing wires installed from the thermostat through the walls. Likewise, replacing an older HVAC system and thermostat connected to each other by a minimum number of wires with a newer HVAC system having more functions may require the installation of more wiring, which can be labor intensive and costly.